Short Message Service (SMS) allows users to exchange text messages between capable mobile phones and other devices within a wireless communications network. SMS is a store and forward service. In other words, text messages are not sent directly from sender to recipient. Rather, text messages are sent to an SMS Center (SMSC). Each wireless communications network can include multiple SMSCs to manage the receipt, storage, and forwarding of text messages.
SMS features message delivery confirmation so that the sender can receive a return message notifying them whether or not the short message has been delivered. SMS messages can be sent and received simultaneously with GSM (Global System for Mobile Communications) voice, data, and fax calls. This is possible because SMS messages are sent using the signaling path and not a dedicated radio channel. As such, users of SMS rarely, if ever, get a busy or engaged signal as they can do during peak network usage times. SMS delivery, however, is not guaranteed. In some instances, the message is lost and never sent to the recipient.
Multimedia Messaging Service (MMS) allows users to exchange multimedia communications between capable mobile phones and other devices. MMS is an extension of the SMS protocol and defines a way to send and receive messages that include images, audio, and video in addition to text.